This paper intends to present and discuss the performance of geogrid reinforced and unrein-forced granular fill layer in unpaved road systems using large scale cyclic plate load tests. A large scale cyclic plate load test facility was developed to study the permanent deformation (rutting) characteristics. Cyclic loads at a constant frequency were applied on geogrid reinforced and unreinforced laboratory un-paved road sections through a rigid circular plate. An unpaved road structure consisting of granular road material over a soft clay soil subgrade was made. To prepare reinforced sections, geogrid was placed within the granular road material at the desired location. Also, geotextile was placed at the interface between road material and soft clay soil subgrade for separation. The model of unpaved road structure was constructed in a steel tank. A total of 5 large scale laboratory tests were conducted to evaluate the effects of geogrid reinforcement. The test results indicated that considerable improvement in bearing capacity was observed when geogrid was placed within the granular road material at all levels of deformations. Permanent defor-mation (rutting), plastic surface deformation and vertical stress development under cyclic loading was greatly reduced with the inclusion of geogrid. The optimum placement position of geogrid was found at a depth of one-third of the plate diameter below the surface.
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